In recent years, as information and communication technologies progress, there is a demand for higher-speed optical transmission, a larger transmission capacity, and a longer transmission distance, which require a considerably advanced transmission technique. A form of such an optical transmission technique is a PON (Passive Optical Network) system. In the PON system, one station and a plurality of subscribers are connected by time-division multiplexing. In upward transmission from each subscriber to the station, the transmission distance from each subscriber to the station varies from subscriber to subscriber, resulting in a significant difference in received power. Particularly, at present, high speed and long distances are required, so that the width of a power level between burst signals is being further increased. In such a situation, for light receiving circuits, it is essentially necessary to output a signal having a stable duty so that a signal can be appropriately received irrespective of the magnitude of the signal and data is reproduced with high precision in a subsequent amplifier circuit or clock recovery circuit.
However, in conventional light receiving circuits, when a large optical signal is input, the output of a transimpedance amplifier is saturated, so that a distortion occurs in a waveform. Also, in worst cases, the output is fixed to an H level or an L level, so that a waveform is not output.
A conventional light receiving circuit used to address this problem is illustrated in FIG. 2.
In the light receiving circuit of FIG. 2, an electrical signal IN100 which is a current obtained by photoelectric conversion of an input optical input signal by a light receiving element 100 is input to an inversion amplifier 101. A feedback resistance R100 is connected in parallel between the input and output of the inversion amplifier 101 to configure a transimpedance amplifier. Further, a series connection of a feedback resistance R101 and a diode RC, a series connection of a feedback resistance R102a and a transistor M100a, and a series connection of a feedback resistance R103a and a transistor M100b are connected in parallel to the feedback resistance R100 of the transimpedance amplifier.
The output of the transimpedance amplifier is input to comparators 102a and 102b, and is compared with comparative values set in the comparators 102a and 102b. These comparative values are a comparative value VB100a in the comparator 102a and a comparative value VB100b in the comparator 102b. 
The comparison results of the comparators 102a and 102b are input to clock input terminals C of flip-flops 103a and 103b whose data input terminals D are connected to a VDD voltage. A signal 100a and a signal 100b which are output from output terminals Q of the flip-flops 103a and 103b are input to the gates of the transistors M100a and M100b, respectively.
With the configuration, when the output values of the comparators 102a and 102b go to the high level, i.e., when an optical input signal has a large level, so that the output value of the transimpedance amplifier is lower than the comparative value VB100a or VB100b set in the comparator 102a or 102b, the comparison results output from the comparators 102a and 102b go to the high level, and a high-level signal is output of the output terminal Q of the flip-flop 103a or 103b which receives the high-level signal at the clock input terminal C.
Thereby, the transistor M100a or M100b is switched ON, so that a value when the feedback resistance R100 and R102a or R102b are connected in parallel to the transimpedance amplifier is fed back to the transimpedance amplifier, and therefore, the amplification factor is suppressed so that the value of the output OUT100 is suppressed to an appropriate value.
The light receiving circuit is described in Patent Document 1. Conventionally, the magnitudes of the feedback resistances are switched in a manner as described above, so that when a large optical signal is input, the feedback resistance is set to be small so that the gain of the transimpedance amplifier is reduced, thereby preventing saturation.
Examples of a means for generating a control signal, depending on the output of the transimpedance amplifier includes: (1) a means for generating a control signal using a feedback-type automatic gain control (AGC) function implemented using an analog circuit; (2) a means for determining the amplitude of the output of the transimpedance amplifier using several comparison circuits as illustrated in FIG. 2, and using the result, generating a control signal using; and (3) a method of determining the amplitude of the output of the transimpedance amplifier using a number of comparison circuits, and using the result, generating a control signal.    Patent Document 1: Japanese Patent Unexamined Publication No. 2000-315923 (FIG. 1).